The present invention relates to an improved apparatus for raising and lowering a transfer arm in a drilling apparatus.
U.S. Pat. No. 4,407,629, assigned to the assignee of the present invention, discloses a drilling apparatus with important commercial advantages. The disclosed drilling apparatus uses a pivoted transfer arm which is mounted adjacent to the drilling mast and is used to move down-hole tubulars between a lower position aligned with a pipe rack and an upper position aligned with the drilling axis of the drilling apparatus. Such transfer arms provide important efficiencies in reducing the manpower and the time required to assemble and disassemble drill strings.
Drilling and casing tubulars are customarily stored on pipe racks at a drill site. It is therefore advantageous for the transfer arm to be designed such that it can load and unload tubulars directly to and from such pipe racks with a minimum of vertical movement of the tubulars. For this reason, it is highly desirable to design the transfer arm positioning system such that tubulars can be transferred between racks and the transfer arm at a relatively low level, typically not over 60-68 inches above ground level.
One prior art approach has been to utilize three or four stage hydraulic cylinders to raise and lower the transfer arm. However, such multiple stage cylinders have a number of important disadvantages. First, such cylinders have been found to provide a relatively short life in many applications. Typically, multiple stage cylinders are designed for intermittent in contrast to continuous duty, and the continuous operations which are necessary when making round trips to change bits, fishing tools or the like can pose excessive demands. Moreover, multiple stage cylinders are not well suited to resist side loads of the type that inevitably occur when rolling heavy drill collars into the clamps mounted on the transfer arm. In addition, multiple stage cylinders expose relatively large hydraulic surfaces which can cause problems when exposed to drilling muds that drip off of drilling tubulars as they are transferred between the drill string and the pipe racks.
Multiple stage cylinders also pose control problems. This is because such a cylinder lifts relatively slowly during the first 10.degree. or 20.degree. of upward movement. However, for a given volume of fluid flow, the angular speed of rotation of the transfer arm increases rapidly as the transfer arm nears the vertical position. This makes it difficult to decelerate the transfer arm smoothly as it approaches the upper position.
A second approach to raising and lowering a transfer arm is shown in the above referenced U.S. Pat. No. 4,407,629. This arrangement utilizes a linkage having a single stage hydraulic cylinder and two fixed length links. This approach also suffers from disadvantages related to smooth control of the deceleration and velocity of the transfer arm as it approaches the upper position. The arrangement shown in U.S. Pat. No. 4,407,629 provides a relatively large angle of rotation of the transfer arm for a relatively small extension of the hydraulic cylinder when the transfer arm is near the vertical position. For this reason, precise control over deceleration and velocity of the transfer arm is difficult to achieve.